1. Field of the Invention
The present invention relates to a marine vessel steering system for a marine vessel with two outboard motors.
2. Description of the Related Art
An outboard motor is an example of a propulsion device for a marine vessel and includes a motor and a propeller driven by the motor. The outboard motor is attached to a stern of the marine vessel in a state enabling turning in the right and left directions. The marine vessel is equipped with a steering apparatus to control a turning angle of the outboard motor. The steering apparatus turns the outboard motor in accordance with an operation of a steering handle by a marine vessel operator. In a case of a multiple installation arrangement in which a plurality of outboard motors are installed at the stern, the steering apparatus turns the plurality of outboard motors in synchronization.
U.S. 2007/0207683 A1 discloses a marine vessel that includes two outboard motors, electric motors to steer the respective outboard motors, and a controller that controls the electric motors. The two outboard motors are aligned and attached along a stern of a hull. The controller changes a relative angle (toe angle) between the two outboard motors at neutral positions (straight positions) in accordance with a traveling state of the marine vessel and, with the changed toe angle, performs a turning angle control in accordance with the steering handle operation.
Here, the toe angle refers to an angle φ defined by mutually straight lines extending along propulsive force directions of the two outboard motors 3P and 3S as shown in FIG. 8A or FIG. 8B. The toe angle indicates whether front ends of the two outboard motors 3P and 3S are directed inward or outward with respect to a heading direction of the marine vessel 1 when the marine vessel 1 is viewed from above. A toe angle in a case where the front ends of the two outboard motors 3P and 3S are directed inward with respect to the heading direction as shown in FIG. 8A is referred to as a “toe-in” angle. A toe angle in a case where the front ends of the two outboard motors 3P and 3S are directed outward with respect to the heading direction as shown in FIG. 8B is referred to as a “toe-out” angle. In the preferred embodiments of the present invention, a toe-out angle shall be expressed as being positive (+) and a toe-in angle shall be expressed as being negative (−).
With the prior art described in U.S. 2007/0207683 A1, a top speed mode (a traveling mode in which the speed is maximized) and an acceleration mode (a traveling mode in which acceleration to a predetermined speed is performed in a minimum time) are defined in advance and the related characteristic engine toe angles are identified and stored in the controller as traveling performance modes. Also, a traveling state detecting device for detecting the speed, acceleration, etc., is provided. When a marine vessel operator selects one mode from among the traveling performance modes prepared in advance, the controller sets a target toe angle based on a target traveling performance corresponding to the selected traveling performance mode, a traveling state detected by the traveling state detecting device, etc. The controller then controls the electric motors so that the toe angle is equal to the set target toe angle.
Operation in a case where the top speed mode is selected shall now be described specifically. A top speed mode map, etc., expressing a relationship between the speed and toe angle in the case where the top speed mode is selected, are stored in advance. When the top speed mode is selected, a toe angle, which, among the toe angles stored in the top speed mode map, corresponds to the speed detected by the traveling state detecting device, is set as the target toe angle and the electric motors are controlled so that the toe angle is equal to the set target toe angle.
With the marine vessel controlled so that the toe angle between the two outboard motors becomes equal to the target toe angle, even during straight traveling of the marine vessel, an external force acting so as to prevent the toe angle from becoming equal to the target toe angle acts on both outboard motors due to a water stream generated at a periphery of the marine vessel. Thus, to maintain the toe angle between the two outboard motors at the target toe angle against the external force during straight travel, the electric motors must be made to generate motor torques of magnitudes enough to cope with the external force. Thus, the required motor torque increases with the external force. Therefore, due to the manner in which an electric motor operates, the current required to supply the electric motor, i.e. the electric consumption, increases when the external force increases.